I've seen three methods:
1. Place them on the same side, sync them.
2. Place one on the side and one on the back, put them in sync, or mess with the gyre mode (not sure if that works with only 2).
3. Put them on opposite sides, but anti-sync them if in the middle of the panels, or put one toward the front and one toward the back and sync them.
What are the dimensions of your 40?
I'm typically a fan of option (and not in the middle of the panel), or 3b, but it depends on the aspect ratio. More square tanks it matters more. Longer tanks it matters less as one end will interfere less with the other.